When the E. coli chromosome is damaged or its synthesis inhibited, a variety of new cellular functions are expressed: these include phage induction, induction of recA protein, new DNA repair ability and mutagenesis. Our objective is to understand how these functions become expressed at the molecular level. The product of the recA gene, which regulates the expression of these inducible functions is a highly specific ATP and polynucleotide dependent protease. We have shown that in crude cellular extracts it cleaves the product of another E. coli gene, lexA, which also regulates the induction mechanism. We wish to study this cleavage reaction and compare it to the cleavage of phage lambda repressor by the recA protease. To determine the intracellular significance of the cleavage reaction, we will study cleavage of mutant lexA proteins or phage repressors produced by a set of host or phage mutants which show an altered response to induction. Mutant recA proteins, including those from some new types of mutants which we propose to isolate in this study, will also be tested. Finally, we will look for evidence of cleavage of the products of other genes and other proteins implicated in expression of SOS functions. The recA protein made in induced cells will be analyzed in a search for one or more biochemical alterations following induction. An in vitro system for measuring fidelity of DNA replication will be established and the influence of recA protein on this system will be studied.